The present invention relates to the current carrying capabilities of superconductors and especially to the fact that the critical currents of many superconductors are limited by grain boundaries formed within such polycrystalline devices. The invention overcomes this limitation of present superconductors by using chemical alterations for improving the current transport properties of the superconductors"" grain boundaries. With the aim of diminishing detrimental effects caused by space charge layers formed at the grain boundaries, this is done by doping the superconductors to dopant concentrations different from concentrations that would provide the optimum superconducting properties of the grains of the superconductors.
Based on the new class of superconductors, henceforth referred to as high-Tc superconductors, which were discovered by Bednorz and Muller and disclosed in their article xe2x80x9cPossible High-Tc Superconductivity in the Baxe2x80x94Laxe2x80x94Cuxe2x80x94O Systemxe2x80x9d, Zeitschrift fxc3xcr Physik B, Condensed Matter, Vol. B64, 1986, pp. 189-193, a variety of superconducting wires, cables and tapes have been developed for the transport of electrical current. A key parameter defining the performance and thus the economic benefit of these conductors is given by their so-called critical current density, which is the maximum density of the current these conductors can carry as supercurrents in the superconducting state. The critical current density is a specific property for a given superconductor and is aimed to be maximized for the practical use of the superconductor.
Chaudhari et al. have taught in their article xe2x80x9cDirect Measurement of the Superconducting Properties of Single Grain Boundaries in YBa2Cu3O7xe2x88x92xcex4xe2x80x9d, Physical Review Letters, Vol. 60, 1988, pp. 1653-1655, that the limiting factor for the critical current density of polycrystalline high-Tc superconductors is the electronic behavior of the boundaries formed by the crystalline grains of these materials. This group has shown that the critical current densities of these grain boundaries is smaller by one to two orders of magnitude than the critical current densities of the grains abutting the grain boundaries.
Further, Dimos et al. have taught in their publication xe2x80x9cSuperconducting transport properties of grain boundaries in YBa2Cu3O7xe2x88x92xcex4 Bicrystalsxe2x80x9d, Physical Review B, Vol. 41, 1990, pp. 4038-4049, that the detrimental effect of the grain boundaries can be reduced by aligning the superconducting grains with respect to their crystalline main axis. Following this proposal, several groups are fabricating wires and tapes of high-Tc superconductors, the critical currents of said wires and tapes are enhanced by aligning the superconducting grains by a variety of means, such as rolling processes or ion beam assisted techniques. Although these technologies have lead to the fabrication of high-Tc superconductors with current densities of the order of 100000 A/cm2 at temperatures of 4.2 K, it remains desirable to fabricate high-Tc superconductors with still higher critical current densities or with processes which are less costly than the known ones.
The present invention now aims in a quite different direction to achieve the desired high current densities in high-Tc superconductors. This different and novel approach according to the invention isxe2x80x94in briefxe2x80x94the teaching that the critical current densities of the grain boundaries in a high-Tc superconductor, and therefore also of the polycrystalline conductors, can be enhanced by appropriate chemical doping of the superconductor.
Sung et al. describe in xe2x80x9cProperties of Doped YBCO Bicrystal Grain-boundary Junctions for Josephson Field Effect Transistorxe2x80x9d, Physica C, Vol. 282-287, 1997, p.2475-2476 and Dong et al. in xe2x80x9cElectric Field Effect in Sm1xe2x88x92xCaxBa2Cu3Oy Bicrystal Junctionsxe2x80x9d, IEEE Transactions on Applied Superconductivity, Vol. 5, 1995, pps. 2879-2882, a method for optimizing the performance of superconducting three-terminal devices based on grain boundaries embodied into the devices. However, it is nowhere in this paper recognized, suggested, or made obvious for the person skilled in the art that the critical current density of the grain boundaries can be enhanced by appropriate doping.
As reported in xe2x80x9cEnhanced Jc and improved grain-boundary properties in Agdoped YBa2Cu3O7xe2x88x92xcex4 filmsxe2x80x9d, Appl. Phys. Lett., Vol. 71, 1997, pp. 137-139, Selvam et al. investigated the effect of adding silver on the critical current density of YBa2Cu3O7xe2x88x92xcex4 films. These researchers found that the addition of silver enhances the size of the superconducting grains and their alignment, and because of these structural changes increases the overall critical current density of the conductor.
Ivanov et al. report in xe2x80x9cProperties of locally doped bi-crystal grain boundary junctionsxe2x80x9d, Physica B, Vol. 194-196, 1994, pp. 2187-2188, experiments in which they investigated the effects of Fe and Pt embedded into grain boundaries in YBa2Cu3O7xe2x88x92xcex4 films on the transport properties of these grain boundaries. In this work, a strong reduction of the critical current density was observed.
Also, the doping of grains in various high-Tc superconductors was reported by several groups, e.g. by M. Muralidhar and M. Murakami in their article xe2x80x9cEffect on Gd Addition on the Superconducting Properties of (Ndxe2x80x94Smxe2x80x94Eu) 123 Systemxe2x80x9d, Applied Superconductivity, Vol. 5, 1997, pp. 127-131, or by T. W. Li et al. in their article xe2x80x9cEnhanced flux pinning in Bi-2212 single crystals by planar defects introduced via Ti-substitution xe2x80x9d, Physica C, Vol. 274, 1997, pp 197-203. However, these efforts are aimed at increasing the pinning forces acting on Abrikosov vortices, which are only present within the grains. Therefore these experiments only affect the superconducting properties of the grains. An effect on the critical current densities of the grain boundaries was not reported, and probably not investigated.
A general objective of this invention is to provide a solution for an increased use of high-Tc materials by a better understanding of the current transport mechanisms within such materials, in particular of the upper limits of the current transport.
A specific objective, as mentioned above, is to provide an approach by which the current carrying capabilities of conductors, e.g. wires, made from high-Tc superconductors can be significantly improved.
Another objective of the invention is to provide a technique for a simpler manufacturing process, resulting in cheaper mass production of polycrystalline high-Tc superconductors with large critical current densities. At present, the fabrication of such superconductors requires cumbersome and costly processes to optimize the grain boundary geometries, such as improving their alignment or enhancing the effective grain boundary area.
A still further objective is to provide an approach for reducing the sensitivity of the critical current densities of polycrystalline high-Tc superconductors to magnetic fields, which often exist in the environment in which the superconductor is operated or which are induced by the supercurrents themselves. At present, the critical current densities of high-Tc superconductors are easily suppressed by such fields, which poses a severe problem for all applications in which the superconductors must be operated in magnetic fields, such as superconducting magnets or superconducting cables.
In brief, the solution taught by the invention is based on a novel understanding of the transport mechanism within polycrystalline high-Tc superconductors and consists in chemically doping the superconductors. This is done to modify space charge layers formed at the grain boundaries, e.g. by changing the density of the superconductor""s charge carriers in an uncommon way. By this, the current transport properties of the grain boundariesxe2x80x94not those of the grainsxe2x80x94within the high-Tc material are optimized and thus the specific objective of this invention achieved.
The herefore used concentrations of dopant ions differ substantially from the concentration levels typically used to fabricate polycrystalline high-Tc conductors. The reason is that the standard approach was to optimize the superconducting properties of the grains and not those of the grain boundaries. The novel approach according to the invention now changes this picture and the understanding of the current transport mechanism in high-Tc materials, especially with regard to the maximum (or critical) current density in such materials.
Another advantage of the approach according to the invention is that it simplifies manufacturing processes and allows cheaper mass production of polycrystalline high-Tc superconductors with large critical current densities, since it reduces the complexity of steps and processes which are often used today for optimizing the geometries of the grain boundaries, as mentioned above.
A still further advantage is the fact that the critical current densities of high-Tc superconductors according to the invention show a reduced sensitivity to magnetic fields which quite often exist in the vicinity of the superconductor, either from other sources or induced by the supercurrents themselves. Thus, superconductors according to the invention can be securely and efficiently operated in environments where strong magnetic fields are abundant, such as superconducting magnets or superconducting cables.